The invention relates to an improved method of producing a modified smectite or smectite-containing substance capable of taking up and releasing water in a reversible manner.
It is well known that the swellability of smectites and smectite-containing substances (such as rocks, mine products and artificial mixtures) can be improved considerably by subjecting them to activation. In this respect activation means that the starting smectite or smectite-containing substance is treated with a salt of a monovalent cation and/or with a magnesium compound. Examples of smectites are montmorillonite, beydellite, hectorite, nontronite, illite, allevardite and palygorskite. A particularly important representative of smectite-containing substances is bentonite. GB 458,240, GB 447,710 and DE 940,936 disclose the use of salts of monovalent cations, such as sodium, potassium, lithium and ammonium salts, as activating agents, whereas according to DE 1,081,436, DE 1,204,995 and DE 1,205,473 a magnesium compound, such as magnesium oxide or magnesium carbonate, is used as activating agent optionally in combination with a salt of a monovalent cation. Activation results in the formation of a highly swellable product which, when admixed with water, forms a thixotropic gel.
European patent application EP 0 645 181 relates to a method of treating a smectite clay that has been recovered and dried to moisture content of about 12% or less, based on the dry weight of the clay, to improve its ability to absorb or adsorb contaminated liquids when in contact with the liquid containing a dissolved salt or an organic contaminant. Such a method comprises several steps, i.e.
impregnating or mixing the clay with dissolved polymer from a an aqueous solution in an amount of at least about 0.5% by weight, based on the dry weight of the clay, and rewetting the clay with water to a moisture content of more than about 12% by weight, based on the dry weight of the clay;
drying the rewetted polymer-impregnated clay to a moisture content of about 12% by weight or less, based on the dry weight of the clay; and
contacting the treated clay with the contaminated liquid.
It is also well known that the activated forms of smectites and smectite-containing substances (e.g. activated bentonite) are able to react with water soluble polymers under forming gels which are no longer thixotropic, have a stable structure, and are capable of taking up and releasing water in a reversible manner (see e.g. EP-A 0,335,653, U.S. Pat. No. 5,120,344, U.S. Pat. No. 5,640,168, U.S. Pat. No. 6,340,385 and references cited therein). The resulting gels are excellent sealing agents and can be used primarily to provide for watertight insulations.
The known methods for producing such gels can be classified into four main groups:
(1) the non-activated smectite or smectite-containing substance is activated first in the presence of water and reacted then with the polymer in the presence of water to form a cross-linked reaction product wherein chemical bonds are formed between the polymer and the elementary lamellae of the smectite (see e.g. HU 186,325 and HU 189,280); or
(2) the non-activated smectite or smectite-containing substance is treated first with the polymer in the presence of water and then the resulting pre-reacted substance is treated with the activating agent in the presence of water to form the final cross-linked structure (see e.g. EP-A 0,335,653); or
(3) as a combination of the above two methods; the non-activated smectite or smectite-containing substance is reacted in a single step, in the presence of water, with both the activating agent and the polymer to form the final cross-linked structure (see e.g. one of the methods disclosed in U.S. Pat. No. 5,120,344); or
(4) the non-activated smectite or smectite-containing substance, the polymer and the activating agent are admixed in the absence of water, and the resulting dry mixture is applied on the surface to be watertightly insulated; the final cross-linked structure forms spontaneously when the mixture contacts with water (see e.g. U.S. Pat. No. 5,604,168).
The first three methods lead to the formation of high quality gels applicable with good results for insulation purposes. They have, however, the common disadvantage that as the formation of the final cross-linked structure proceeds, a very intense kneading of extremely high mechanical energy demand is required. The resulting moist hydrogel is difficult to store and transport. Therefore the references disclosing methods (1) to (3) above suggest to perform either the whole process or at least its final step directly at the place of use. However, the specific preconditions (e.g. specific equipments, energy source, skilled manpower etc.) of such a direct production can not always be met. To obtain a xerogel which is easy to store, transport and manipulate at the place of use the moist hydrogel is to be dried first, which is lengthy and requires much energy. These disadvantages render the methods less attractive from economical points of view.
The above difficulties can be avoided by using method (4). This method involves, however, the disadvantage that the insulating ability and adhesive power of the gel which forms spontaneously at the place of use highly depends on the prevailing environmental conditions which cannot be influenced. From the aspects of reliable product quality a fully controlled operation series is much more appropriate than one including a spontaneous step.